As is known, a single radar system having a radar antenna, also referred to herein as a radar array, has a theoretical maximum processing gain and signal to noise ratio, each of which directly affects the ability of a radar to detect and to track a target. The maximum processing gain and the maximum signal to noise ratio are determined by a variety of radio characteristics, including but not limited to, a radar antenna size, a radar transmit and receive beamwidth, a type of received signal processing, a radar transmit power, and a radar receiver noise. Each of these characteristics is substantially fixed for any given radar system. Therefore, in order to improve detection and tracking performance, it has generally been necessary to design a new radar system having new characteristics.
Alternatively, it is possible to process together the received signals from a plurality of radars, each having a radar antenna, in order to increase processing gain, and therefore, to increase detection and tracking performance. In order to process together the received signals, i.e., the target echoes, from the plurality of radars, it is advantageous that received signals associated with each respective one of the plurality of radars be processed together at the same phase, i.e., coherently. It would also be advantageous if the processing gain provided by the plurality of radars approaches an ideal coherent processing gain. However, since the antennas of different ones of the plurality of radars are physically separated, the signals they receive as echoes from a target are generally not in phase, and therefore, do not combine coherently.
One of ordinary skill in the art will understand that knowledge of the relative position of the radar antennas of each respective one of the plurality or radars to within a small fraction of a wavelength of the received radar signals allows time delay (and phase) corrections in the transmitted and received signals to be made having sufficient accuracy to allow nearly ideal coherent processing. However, it is generally not sufficient that the position of the radar arrays merely be mechanically measured, since the distance between the radar arrays can be quite large compared to a radar signal wavelength, resulting in measurement inaccuracy. Furthermore, mobile radars are subject to changes in relative position much greater than a wavelength, and therefore, calibration of relative position would have to be performed each time the mobile radars are moved.
Also, radar systems can introduce relative time delay differences according to different time delays of their respective transmit and receive electronics, which can result in substantial time delay differences between radar systems, also resulting in lack of signal coherency between radars.
One of ordinary skill in the art will understand that calibrating the relative positions and relative time delay differences of a plurality of radar arrays is difficult and subject to increasing errors as the separation of the plurality of radars increases. It will also be understood that the calibration can be performed in a separate process, requiring time apart from actual operation of the radars.